Multiple Small Subs - Geddes Approach - Page 158 - diyAudio
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Old 17th February 2013, 07:56 PM   #1571
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Quote:
Originally Posted by Brian Steele View Post
I've actually measured dips in FR caused by my test TH being left unpowered and unconnected in my living room. Even published them somewhere, I think. Purposely designing one for this purpose may work.
Found it - TH as Bass Trap?
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Old 17th February 2013, 09:15 PM   #1572
Moonfly is offline Moonfly  Spain
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I've seen reports of unpowered subs absorbing bass, so it does occur, but why wouldnt you just use variable tune passive radiators.
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Old 17th February 2013, 10:39 PM   #1573
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I have never seen research on it, but I believe a motional feedback woofer will eat any sound that hits it. Or maybe it will amplify it.

Ben
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Old 17th February 2013, 10:46 PM   #1574
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MFB does the opposite - it prevents the cone from moving in response to external influence. (The backpressure of a small enclosure is an "external influence".)
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Old 17th February 2013, 10:52 PM   #1575
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Originally Posted by Don Hills View Post
MFB does the opposite - it prevents the cone from moving in response to external influence. (The backpressure of a small enclosure is an "external influence".)

Well then, there must be something in that family that does the trick?

How about an amp with a positive impedance equal to the driver? (In contrast to an MF amp whose output impedance is the negative of the driver.)

OK... sounds distinctly unstable.

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Old 18th February 2013, 12:04 AM   #1576
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If you arranged an amp/driver so that any external influence causing cone movement was amplified instead of canceled, it would act as a bass trap. You can prove the concept by using two subs, both driven but one out of phase with the other, and moving them about.

I think Brian and Patrick are on a more promising track, though. Broadband bass absorption / cancellation is, in my opinion, counterproductive. You don't want to reduce all bass equally - you may as well just turn down the bass in the first place. Trapping / canceling individual resonances is a more fruitful approach. The classical way is to use a 1/4 wave tube resonator placed at, and tuned to, the location and frequency of the mode to be canceled. A 1/4 wave tube is the basis of a tapped horn, so it might be possible to actively drive the tapped horn to amplify the resonance and increase the canceling effect.

... I see I spoke up in the "TH as trap" thread too...
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Old 18th February 2013, 02:26 AM   #1577
AllenB is offline AllenB  Australia
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Originally Posted by Don Hills View Post
Broadband bass absorption / cancellation is, in my opinion, counterproductive. You don't want to reduce all bass equally - you may as well just turn down the bass in the first place.
I don't see this being the case. If the walls were transparent to bass there'd be no room and no modes, and a fixed bass trap could in theory achieve the same thing to some degree....Where bass doesn't align itself to a specific mode, it will fall into the background but modal bass will continue for a time where both the peaks and dips could be accessed at some point, perhaps by a trap of some sort. The multi sub approach is just the same thing, but active. However, peaks and nulls can have modal energy added to/subtracted from them through various modes as well as being able to fight them in the source modes as an absorber would.
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Old 18th February 2013, 02:20 PM   #1578
gedlee is offline gedlee  United States
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Originally Posted by Patrick Bateman View Post
Dr Geddes - I know you've done a lot of research into bass traps and room treatment. You were the first person to clue me into the idea of using Helmholtz bass traps.

I have a question -

Can one use unpowered sealed subs as bass traps?

For instance, one could construct a helmholtz bass trap, but calculating the volume of the box and the volume of the helmholtz resonator is a mystery to me. (Is it the same math as a vented box? I do not know.)

But it occurred to me that one could simply build a sealed box, pick a woofer with the proper FS, and leave the box unpowered. It seems to me that the woofer cone would act like a passive radiator, and would cancel out sound in the room at that frequency.

Thoughts?
You could do that, but it would not be all that effective. Better is to use an amp and some electronics control to create an active absorber which would be much more effective, but better still is just to use the sub as a sub in a multi-sub setup. If done correctly, you will find that turning a sub on can actually reduce the sound level in some frequency range. Hence this sub IS acting as an absorber in that frequency range. You will find that all the subs do this over some frequency range. This means that a multi-sub setup IS an active absorber setup.

Unless the absorption is built into the room walls its not going to be very effective - multi-subs simply works and trumps all of the other approaches IMO.
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Old 18th February 2013, 02:31 PM   #1579
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Originally Posted by gedlee View Post
You could do that, but it would not be all that effective. Better is to use an amp and some electronics control to create an active absorber which would be much more effective, but better still is just to use the sub as a sub in a multi-sub setup. If done correctly, you will find that turning a sub on can actually reduce the sound level in some frequency range. Hence this sub IS acting as an absorber in that frequency range. You will find that all the subs do this over some frequency range. This means that a multi-sub setup IS an active absorber setup.

Unless the absorption is built into the room walls its not going to be very effective - multi-subs simply works and trumps all of the other approaches IMO.
Now if we only had an active absorption technique that could tame modal decay with more precision, i.e. DBA/CABS without the strict placement requirements for low frequency sources.
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Old 18th February 2013, 04:56 PM   #1580
gedlee is offline gedlee  United States
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Markus

Again, I would contend that the modal decay has to be reduced if the SPL level is made flat, no matter how that is achieved. The system is far more minimum phase than it is not minimum phase.
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